Since the announcement of the first commercial installations of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining", volume 4, page 433 (Interscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics, separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point. .degree.F. Point .degree.F. Gal. ______________________________________ Ethylene benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 292.0 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources of the mixtures of C.sub.8 aromatics are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 60 wt. % ethyl benzene (EB) with the balance, xylenes, being divided approximately 50 wt. % meta and 25 wt. % each of para and ortho.
In turn, calculated thermodynamic equilibria for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % Ethyl benzene 8.5 Wt. % para xylene 22.5 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 TOTAL 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethyl benzene by about 1 wt. %, will not change the ortho-xylene, and will decrease the para and meta xylenes each by about 0.5 wt. %.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward a equilibrium mix and thus increasing yields of the desired xylenes.
The Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.
______________________________________ Process Conditions ______________________________________ Reactor Pressure 175 to 225 PSIG Reactor Inlet Temperature Range 830-900.degree. F. Heat of Reaction Nil Liquid Hourly Space Velocity 0.6 to 1.6 Vol/Vol/Hr. Number of Reactors, Downflow 1 Catalyst Bed Depth, Feet 11 to 15 Catalyst Density, Lb/Cu. Ft. 38 Recycle Circulation, Mols 7.0 to 14.0 Hydrogen/Mol Hydrocarbon Feed Maximum Catalyst Pressure Drop, PSI 20 ______________________________________
It will be seen that the Octafining system is adapted to produce maximum quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethyl benzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining has been unique in its ability to convert ethyl benzene. Other xylene isomerization processes have required extremely expensive fractionation to separate that component of C.sub.8 aromatic fractions. As will be seen from the table of properties above, the boiling point of ethyl benzene is very close to those of p- and m-xylene. Complete removal of ethyl benzene from the charge is impractical. The usual expedient for coping with the problem is an ethyl benzene separation column in the isomerizer-separator loop when using catalysts other than those characteristic of Octafining. It will be seen that Octafining does not have this expensive auxiliary to prevent build up of ethyl benzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethyl benzene.
The Octafining process has been extensively discussed in the literature, for example:
1. Pitts, P. M., Connor, J. E., Luen, L. N., Ind. Eng. Chem., 47, 770 (1955). PA0 2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955. PA0 3. Ciapetta, F. G., U.S. Pat. No. 2,550,531 (1951). PA0 4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189. PA0 5. Octafining Process, Process Issue, Petroleum Refinery, 1st Volume 38 (1959), No. 11, Nov., page 278.
A typical charge to the isomerizing reactor (effluent of the crystallizer) may contain 17 wt. % ethyl benzene, 65 wt. % m-xylene, 11 wt. % p-xylene and 7 wt. % o-xylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasible, consistent with reaction times which do not give extensive cracking and disproportionation.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethylene and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethyl benzene include C.sub.6 +naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation and total loss to other than C.sub.8 molecular weight. C.sub.5 and light hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include napthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity and that effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A relatively recent development in this art involves the use of a unique class of zeolite catalysts for isomerization of xylenes in a p-xylene recovery loop. The zeolite catalysts designated ZSM-5 and ZSM-12 as well as other zeolites having like properties will induce extensive disproportionation of ethyl benzene at very low loss of xylene by that reaction, all as described in U.S. Pat. No. 3,856,872, Morrison, dated Dec. 24, 1974. As shown in that patent, isomerization of C.sub.8 aromatics with such zeolite catalysts avoids buildup of ethyl benzene in the loop by converting that compound to lower boiling benzene and higher boiling polyalkyl benzenes which are separated by inexpensive splitters and strippers in the loop.
Another solution to the ethyl benzene problem, in addition to Octafining and the Morrison process, has been to supply xylenes which are free of ethyl benzene. The favored sources of such pure xylene streams are techniques for conversion of toluene as by disproportionation and methylation.
Disproportionation of toluene can be accomplished with porous acid solid catalysts to yield benzene and a mixture of xylenes. The product is, of course, free of ethyl benzene. See, for example, U.S. Pat. No. 3,578,723, Bowes and Wise, dated May 11, 1971.
Reaction of toluene with a methylating agent such as methanol produces xylenes and higher boiling polymethyl benzenes which are readily separated from the produce xylenes and may be reacted with toluene to form additional xylenes by transalkylation reactions. Recent developments in synthesis of xylenes by methylation of toluene have been constituted by provision of catalysts which favor production of p-xylene such that the product xylene streams contains a proportion of p-xylene much in excess of the thermodynamic equilibrium value, thereby facilitating separation of p-xylene at reduced cost. These catalysts having enhanced capability for formation of p-xylene generally manifest a restriction of rate of diffusion of xylenes other than the para isomer, a property conveniently measured as rate of diffusion of o-xylene as set out more fully hereinafter. Patents describing methods for preparation and use of such catalysts include: U.S. Pat. No. 3,965,207 to Weinstein, U.S. Pat. No. 3,965,208 to Butter & Kaeding, U.S. Pat. No. 3,965,209 to Butter & Young, and U.S. Pat. No. 3,965,210 to Chu.
U.S. Pat. No. 4,159,282 to Olson et al describes a xylene isomerization process in which a specified crystalline aluminosilicate zeolite characterized by a crystal size of at least about 1 micron is employed as an isomerization catalyst. In a more specific embodiment, the reaction is carried out with a crystalline aluminosilicate catalyst having a bimodal crystal size distribution generally falling in two ranges, less than about 1 micron and greater than about 1 micron with the latter being in major proportion.
The zeolite preferably has a defined xylene sorption capacity of greater than I gram/100 grams of zeolite and a defined ortho-xylene sorption time for 30 percent of the sorption capacity of greater than 10 minutes, the sorption capacity and sorption time being measured at 120.degree. C. and a xylene pressure of 4.5.+-.0.8 mm of mercury. The defined sorption time is obtained by modifying the zeolite, such as by combining the zeolite with a difficultly reducible oxide, or by precoking or by steaming the catalyst. The original alkali metal of the zeolite may be replaced by ion exchange with other suitable ions of Groups IB to VIII of the Periodic Table. The process employs an ethyl benzene feed containing xylenes. Olson et al broadly state that they can process a mixture of C.sub.8 aromatics, such as that derived from platinum reforming of a petroleum naphtha, to a mixture of reduced ethyl benzene content and increased content of para-xylene. The process, however, results in xylene loss, as shown by the various examples of Olson et al.
U.S. Pat. No. 4,163,028 to Tabek et al describes a xylene isomerization process in which a low acid activity catalyst, typified by zeolite ZSM-5 of low alumina content (SiO.sub.2 /Al.sub.2 O.sub.3 of about 500 to 3000 or greater) and which may contain highly dispersed metals, such as platinum or nickel, is employed as an isomerization catalyst at a temperature of 800.degree. F. or higher. The patent states that at these temperatures, ethyl benzene reacts primarily by dealkylation to benzene and ethane rather than by disproportionation to benzene, and hence is strongly decoupled from the catalyst acid function. Since ethyl benzene conversion thereby is less dependent on the acid function, a lower acidity catalyst can be used to perform the relatively easy xylene isomerization. The patent does not quantify or define the low acid activity of the catalyst or the dispersing or dehydrogenation activity of the dispersed metal. The process permits high conversion of ethyl benzene to benzene and it is stated in the patent to do so at little or no conversation of xylenes. All of the examples in the patent, however, show xylene loss. The process also is stated to be able to convert paraffin hydrocarbons.
U.S. Pat. No. 4,101,595 to Chen et al describes a process for converting ethyl benzene to para xylene which employs a dual function catalyst comprised of a shape selective moderate or low acid zeolite and a strong hydrogenation/dehydrogenation metal of Group VIII of the Periodic Table. The zeolite must have restricted diffusion of o-xylene as determined by a defined ortho-xylene sorption time for 30 percent of the sorption capacity of greater than 10 minutes, which can be achieved by using large crystals of, for example, zeolite ZSM-5 having average dimension of individual crystals about 0.5 microns and greater. This patent states that small crystal zeolites may be modified to show the restricted diffusion effect by techniques known to the art.
Chen et al further state that the zeolite should have a reduced acid function to minimize losses to hydrocracking. Chen et al disclose that this can be achieved by adding a catalyst poison to the charge or by treating the catalyst to reduce its activity by steaming, partial exchange with such cations as alkali metals, partial coking and like known deactivating methods. When operating without a catalyst poison in the charge, Chen et al state that the zeolite should be deactivated to a reduced activity, measured as alpha value, between 0.05 and 1.
The zeolites employed by Chen et al are stated to have unusually low alumina contents, that is, high silica to alumina ratios. The highest silica to alumina ratio disclosed by Chen et al is 200.
Chen et al state that the feed or charge can be a mixture of ethyl benzene and xylene isomers, and that the product stream can contain an increased xylene and decreased ethyl benzene content as compared with the charge. Chen et al state that a feed constituted by an approximately equilibrium mixture of ethyl benzene and the three xylene monomers can be passed to an ethyl benzene converter in which the ethyl benzene is converted in large measure to xylenes, including a proportion of p-xylene in excess of equilibrium. The various examples of Chen et al employ a charge of ethyl benzene and no example shows a mixed charge of ethyl benzene with xylenes. The Chen et al examples for converting ethyl benzene employ a ZSM-5 crystalline having a 2 micron average crystal size and a silica/alumina mol ratio of 70.